1. Field of the Invention
The present invention relates to a terbium-activated oxide phosphor which exhibits green luminescence under excitation with an electron beam, X-rays, ultraviolet rays, etc., and a cathode-ray tube employing such a phosphor as a fluorescent layer.
2. Discussion of Background
Along with the trend for large size televisions, projection-type televisions have become popular. Cathode-ray tubes used for the projection-type televisions are required to have high luminance for enlarged projection, and they are operated at a high current density as compared with cathode-ray tubes for ordinary televisions. Accordingly, the phosphors used for the cathode-ray tubes for the projection-type televisions are usually required to have a good current luminance characteristic (.gamma.-characteristic), be free from deterioration against excitation for a long period of time and have a good temperature characteristic. Further, in recent years, as refinement of cathode-ray tubes advances, it has been required to reduce the spot of the electron beam for each color more than ever. For the phosphors, this means that the current density increases and the load increases. Therefore, phosphors which are less susceptible to deterioration than the currently available phosphors, are desired.
At present, as phosphors for projection cathode ray tubes, a Y.sub.2 O.sub.3 :Eu phosphor for red, a Y.sub.3 (Al, Ga).sub.5 O.sub.12 : Tb phosphor, a InBO.sub.3 :Tb phosphor, a Y.sub.3 Al.sub.5 O.sub.12 : Tb phosphor, a Y.sub.2 SiO.sub.5 :Tb phosphor, a LaOCl: Tb phosphor, etc., for green, and a ZnS:Ag,Al phosphor for blue, are used.
The above-mentioned Y.sub.3 (Al,Ga).sub.5 O.sub.12 :Tb phosphor is excellent in the current characteristic of luminance such that the luminance increases in proportion to the current density in a low current density range (not higher than 10 .mu.A/cm.sup.2) ) and has excellent temperature characteristics such that the luminance undergoes little change against the temperature change of the phosphor, and it is a phosphor having high luminous efficiency.
However, this Y.sub.3 (Al,Ga).sub.5 O.sub.12 : Tb phosphor is not necessarily satisfactory with respect to the life characteristic at a high current density, and a further improvement of the life characteristic has been desired. In order to prevent deterioration of luminance of the phosphor and to prevent saturation of luminance at a high current density, a phosphor has previously been proposed in which a part of Y in the phosphor matrix is substituted by Yb, Tm, Sm, Eu or the like (Japanese Unexamined Patent Publication No. 289679/1990).